The present invention relates to arc plasma torches, and more particularly to a method and a mechanical/pneumatic arrangement for contact starting an arc in such torches.
There are three currently known methods to initiate a plasma arc discharge and start an arc plasma torch: high frequency or high voltage discharge, contact starting, and with an exploding wire technique. In each method, an arc is drawn between a cathode and an anode, and an ionizable gas is directed to flow around the arc, creating a plasma jet.
The oldest, and still most widely used method in the prior art is the high frequency discharge or high voltage spark discharge method. In either case, a high voltage breaks down the gap between a cathode and an anode, generating charge carriers which create the electric current path necessary to start the arc. An example of this method is disclosed in U.S. Pat. No. 3,641,308 to Couch Jr. et al. In that patent, a brief high voltage pulse is provided to the cathode which initiates an arc discharge across the gap from the cathode to a grounded workpiece.
Until recently, the high frequency method of arc starting was used exclusively for plasma torches. However, such an arc starting circuit produces significant amounts of electromagnetic interference in nearby electronic equipment, requiring either extensive shielding or remotely locating the electronics. Furthermore, the equipment required to generate the high frequency discharge tends to be quite expensive and has had a negative impact on the economic competitiveness of plasma cutting torches when compared with other cutting methods.
The second known starting method is contact starting. Although initially developed by Creare Inc. for the Air Force in the early 1960's, this method is just beginning to gain in popularity. Contact starting uses low electric currents, and eliminates the need for high frequency equipment along with the associated high cost and electromagnetic interference. The present invention relates to a novel variation in contact starting, as more fully discussed below, and shares in the known benefits of contact starting.
In all known torches which use contact starting, the cathode is manually placed into electrical connection with the workpiece, a current is passed from the cathode to the workpiece, and the arc is struck by manually backing the cathode away from the workpiece. In several prior art systems, the cathode is the electrode, and the nozzle through which the plasma jet passes serves as an electrical conductor connecting the electrode with the workpiece. In these systems, the nozzle is slidable with respect to the electrode, and is forced into contact with the electrode when it is pressed against the workpice. Thus, the electrode, nozzle, and workpiece are all in electrical series connection when the current flow is initiated. When the electrode is manually backed away from the workpiece, the nozzle is allowed to separate from the electrode and return to its normal position.
These contact starting systems are disadvantageous since the nozzle is pressed against the workpiece. Since there is usually a brittle ceramic element at the end of the nozzle, such pressing risks damage to the nozzle. Also, it is difficult in practice to initiate a cut while at the same time attempting to press the nozzle down onto a workpiece. Another significant problem with this starting method is that nonconductive coatings such as paint makes electrical contact starting using the workpiece very difficult. As a result, most manufacturers today use a pilot arc circuit even when they have contact starting in order to cut through the paint layer.
The third known method of arc plasma torch starting is known as the "exploding wire" technique. An electrical conductor is extended from the cathode to the workpiece. The conductor vaporizes when the current is initiated, leaving the arc in its place. An obvious drawback to the exploding wire technique is, of course, that it is not practical for use in restartable plasma torches.
It is therefore a principal object of the present invention to provide an arc plasma torch that can be started without high frequency discharge or high voltage spark discharge.
Another principal object of the present invention is to start arc plasma torches without electro-magnetic interference in nearby electronics.
It is still another object to provide a torch which can be started away from the workpiece.
A further object is to provide a torch which may be easily restarted after shutdown.
It is still a further object to provide a torch which automatically draws an arc.
Yet a further object is to provide the torch which can be constructed relatively simply and economically, can be operated without expensive external electronics, and can be readily and easily repaired.